Nitric acid is produced industrially in large volumes through the oxidation of ammonia in air over a catalyst, usually platinum, and the subsequent absorption of the nitrogen oxides in water. The "weak" nitric acid produced in such a process normally has a strength about 55% to 65% HNO.sub.3, the balance being water. It is not possible to concentrate this acid through simple distillation beyond about 68% HNO.sub.3, since an azeotrope exists at that concentration. "Strong" nitric acid of strength of above 95% HNO.sub.3 is required in some important industrial processes, such as certain nitration and oxidation processes, and to obtain nitric acid of this strength dehydrating agents are commonly employed which "break" the azeotrope. Dehydrating agents function by preferentially "binding" the water thereby leaving the vapor above the solution richer in nitric acid. It is known that normally a minimum proportion of the dehydrating agent is needed to "break" the azeotrope.
The use of sulfuric acid as a dehydrating agent in nitric acid concentration is well established, but many other dehydrating agents have been proposed and a number have found commercial application. Examples of dehydrating agents described and claimed in the patent literature are calcium nitrate and magnesium nitrate (R. Wolfenstein and O. Boeters U.S. Pat. No. 864,217); magnesium nitrate (A. P. Beardsley U.S. Pat. No. 2,463,453); and lithium nitrate (A. W. Yodis U.S. Pat. No. 3,433,718). To facilitate the following discussion reference is usually made to sulfuric acid as the dehydrating agent, although such other agents can also be used.
In a simple batch distillation process of a suitable mixed acid consisting of sulfuric acid, nitric acid and water, as nitric acid is removed by vaporization from the mixed acid its weight fraction in the liquid phase decreases while that of water increases. This results in the progressive decline in the strength of the nitric acid vapor produced until recovery becomes uneconomical. Compared to batch distillation the economy of nitric acid concentration and its recovery are greatly improved if the mixed acid is passed countercurrently through a packed column with steam rising from the bottom, as disclosed for example in H. Pauling U.S. Pat. No. 1,031,864. The steam condenses as it rises in the column thereby vaporizing nitric acid from the mixed acid. Strong nitric acid is obtained as vapor at the top of the column, while boiling sulfuric acid of about 68% to 72% H.sub.2 SO.sub.4, containing only traces of nitric acid, is discharged at the bottom. The live steam injected into the column supplies the energy for the process in an economical manner, i.e. the heat to vaporize the nitric acid and the heat required to bring the sulfuric acid to its boiling point, but it also dilutes the sulfuric acid and thus increases the amount of "strong" sulfuric acid required for the process. This is because the strength of the spent sulfuric acid at the bottom of the column must not be allowed to drop much below 68% H.sub.2 SO.sub.4. If the spent sulfuric acid is allowed to drop below this strength, an increasing amount of nitric acid will stay with this spent sulfuric acid and will ultimately be lost as this spent sulfuric acid is reconcentrated. The reason for this is that the dehydrating capacity of sulfuric acid decreases rapidly as the spent sulfuric acid strength falls below 68% H.sub.2 SO.sub.4.
"Live steam" usage can be reduced if the nitric and sulfuric acids are fed separately into the concentration column, so that the heat of mixing between these two acids is released within the column; such a process has been disclosed in H. Pauling U.S. Pat. No. 1,074,287. It is possible to avoid "live steam" injection altogether by using steam heated boiling tubes, as disclosed in S. F. Spangler U.S. Pat. No. 1,895,012. The benefits of indirect heating are, however, only of marginal value since the energy requirements of the nitric acid concentration process are relatively low, particularly if the heat of mixing is liberated within the column. Moreover, although it is preferred to avoid dilution of the sulfuric acid for the reason given above, the dilution due to "live steam" injection is relatively small since most of the water absorbed by the sulfuric acid comes from the water that enters the process with the "weak" nitric acid. In modern nitric acid concentration plants energy requirements ar normally met by steam heated boiling tubes (which are usually of high corrosion resistant and expensive metal such as tantalum) often in conjunction with some "live steam" injection. Air injection into the bottom of the concentration column is also practiced, and has the effect of lowering temperatures throughout the column. A review of industrial practice of nitric acid concentration is given in a paper by D. Gericke in Chemie Ingenieur Technik, Volume 21/74, pages 894-899.
Passing of the sulfuric and nitric acids into the concentration column at the proper strengths and in the proper mass ratio is important for efficient use of sulfuric acid and for good yield of nitric acid, but it is by itself not a guarantee that "strong" nitric acid will be produced and that the spent sulfuric acid will contain only small amounts of residual nitric acid; the energy requirements of the process must also be met exactly. Energy input in excess of process requirements through the boiling tubes or through "live steam" injection could lead to a situation where nitric acid of a strength below that specified is produced, while the strength of the spent sulfuric acid increases above the normal operating range of 68%-72% H.sub.2 SO.sub.4. This is because steam is forced up through the concentration column and leaves with the nitric acid vapor at the top of the column thereby diluting the "strong" nitric acid. Insufficient energy input could lead to a situation where "strong" nitric acid might be produced, but where part of the nitric acid stays with the spent sulfuric acid; the spent sulfuric acid is said to be not properly denitrated. The operation of a nitric acid concentration column requires good control of the acid feed rates into the concentration column and of the energy supplied to the process. In industrial practice process conditions are commonly monitored by recording tops and bottoms temperatures of the concentration column in addition to process feed rates and energy input to the process. The composition of the product "strong" nitric acid and of the spent sulfuric acid are also monitored.
The strength of the nitric acid produced in a nitric acid concentrator is commonly controlled in one of two ways, namely by refluxing part of the concentrated "strong" nitric acid to the top of the column or by injecting "strong" sulfuric acid into the column at an elevation above that of the "weak" nitric acid feed point. Only a small amount of reflux of "strong" nitric acid is required, and this reflux converts the top of the nitric acid concentration column into an enriching section operated above the azeotrope of nitric acid. The injection of the "strong" sulfuric acid at an elevation above that of the "weak" nitric acid feed point puts the sulfuric acid of highest concentration, which exhibits the highest relative volatility for nitric acid, into contact with the "strong" nitric acid vapor discharged at the top of the concentration column. However, in commercial practice ideal equilibrium conditions are far from being reached, and in many plants control of the strength of the product "strong" nitric acid is achieved through a combination of the above two methods. Refluxing of part of the "strong" nitric acid has the additional benefit of condensing and collecting trace quantities of sulfuric acid vapor.
At the bottom of the column the strength of the spent sulfuric acid must be kept in the range of about 68% to 72% H.sub.2 SO.sub.4. A lowering of this strength, which would be obtained if the "strong" sulfuric acid feed rate into process is reduced in relation to the "weak" nitric acid feed rate, will lead to unacceptably high levels of nitric acid in the spent sulfuric acid, and that constitutes a yield loss. The reason for this loss in nitric acid is the fact that the relative volatility of nitric acid decreases rapidly if the strength of the spent sulfuric acid is allowed to drop below a strength of about 68% H.sub.2 SO.sub.4. For a column of a given packing height a decrease in the spent sulfuric acid strength leads to a higher residual nitric acid level in the spent acid, even if the energy input is adequate. An increase in the steam rate to process does not necessarily help the situation since the spent sulfuric acid could be further diluted. It is quite possible, through energy input in excess of the process requirements and through improper feed rates to process, to produce nitric acid of unacceptably low strength, while at the same time producing a spent sulfuric acid containing high levels of residual nitric acid.
A major expense in nitric acid concentration is the disposal or reconcentration of the spent sulfuric acid. If sulfuric acid is used on a once-through basis, then storage facilities must be provided for the strong and the spent sulfuric acids, and further expenses are incurred in shipping them. Moreover, the once-through approach ties the nitric acid concentration plant to sulfuric acid supply and disposal facilities. On the other hand, sulfuric acid reconcentration and recycle of this acid in a closed sulfuric acid loop is an energy intensive process, in that not only must the water absorbed by the spent sulfuric acid be removed through vaporization, but in that an amount of energy equivalent to the heat of mixing must be provided. The availability of the heat of dilution or the heat of mixing in the nitric acid concentration column makes this part of the process a relatively low energy consumer, but the penalty is paid in the sulfuric acid reconcentration process, where an equivalent amount of energy must be supplied to again separate sulfuric acid and water by preferentially boiling off the water. Furthermore, there is little opportunity for energy recovery since both the spent sulfuric acid produced in the nitric acid concentration column and the "strong" sulfuric acid discharged from a vacuum sulfuric acid concentrator are at about the same high temperature. The reconcentrated "strong" sulfuric acid must be cooled before being recycled to the nitric acid concentration column, since feed of the hot acid could cause excessive energy input to the nitric acid concentration column with consequences already discussed. Cooling of the sulfuric acid anywhere in the sulfuric acid loop implies that energy is rejected from the process, which must again be resupplied somewhere else in the process. To make matters worse, this energy transfer in the process must usually be implemented using very expensive tantalum equipment. Steam requirements for the nitric acid reconcentration process including sulfuric acid concentration are given in the above-mentioned paper by Gericke as 3.5 tons of steam per ton of "strong" nitric acid for the case of a feed nitric acid of 55% HNO.sub.3. In processes where the sulfuric acid is used on a once-through basis, the requirements of "strong" sulfuric acid are of the order of 2.5 tons per ton of nitric acid; this latter figure varies somewhat depending on the strengths of the feed "weak" nitric acid and of the "strong" sulfuric acid used in the process.
The focus of most prior art has been on the development of an efficient nitric acid concentration process, and the associated problem of sulfuric acid reconcentration has been relatively neglected. Sulfuric acid has a high boiling point as compared with nitric acid and reconcentration of sulfuric acid is notoriously difficult. For this reason, the use of 80% H.sub.2 SO.sub.4 in place of the commercial 94% H.sub.2 SO.sub.4 is advocated in U.S. Pat. No. 1,074,287 (also to H. Pauling), since reconcentration to 80% H.sub.2 SO.sub.4 is much easier than to 94% H.sub.2 SO.sub.4, mainly because of the lower boiling point of the former. Even when concentration is carried out under vacuum relatively high temperatures are necessary, requiring the use of high pressure steam in the boiling tubes and of expensive corrosion resistant heat transfer surfaces.